In fabricating semiconductors, silicon wafers are often held in a cassette and then moved to various pre-programmed processing locations by a robotic handling system. The latter typically includes a mechanism with degrees of freedom in radial (R), angular (.crclbar.) and vertical (Z) directions and having a robot arm with a vacuum or edge-gripping wand. The robot must be able to pick up wafers from a storage cassette and then transfer them to a designated station or a plurality of stations where the wafer will undergo some arbitrary process such as heating or alignment. In order to perform these actions, the robot must have precise knowledge of the R, .crclbar. and Z positions of the wafer at all cassette and station locations. A robot control system must provide the aforesaid knowledge to position the robot arm and thus the gripped wafer precisely within a cassette or process station for each robot function.
In a typical wafer handling layout the general geometry of both the robot and the various process stations such as the cassette stand are known, and the dimensional relationships between the robot and each station are known within nominal tolerances (e.g.=0.05 inches) available from CAD drawings or manual measurements. When in use, however, the robot must be controlled to move wafers within extremely close tolerances in order to assure that the robot system operates properly without damaging any system component or the wafer being handled.
To provide proper operation of the robot system when initially set up or when restarted after replacement of a component or when a process location has been changed, the robot must be programmed or "taught" so that for each operation phase, the robot arm is positioned precisely at the proper location for the desired function. Heretofore, this initial and/or subsequent programming or "teaching" step was accomplished manually by trained personnel using visually estimated trial and error adjustments of the robot mechanism and control.
For example, using conventional controls, a robot was heretofore installed and "taught" by jogging the robot around and, at each process station, recording wafer placement locations with a teach pendant. Besides consuming many hours, this manual procedure introduced subjectivity and thus a significant possibility for errors since no two technicians could set the same positions. This created a problem of reproducibility, that is, of setting the robot in a precise predetermined position for each of a multitude of cycles. Whenever a wafer cassette is not perfectly positioned within specifications or a machine component wears, settles or malfunctions and requires replacement, the robot must be re-taught because it cannot adapt to such variations. If the robot is not re-taught properly within close tolerances, serious damage or loss of expensive wafers could result.
In copending U.S. patent application Ser. No. 09/270,261, filed Mar. 15, 1999, an automatic calibration system for robots is disclosed wherein sensors were employed on both the robot and also on the cassette or process station in which the wafer was to be placed. Although the aforesaid system has been successful in solving the automatic calibration problem, the necessity to provide sensors tended to increase the cost of the system as well as entailing other disadvantages. For certain applications the aforesaid system also provides superior results but in other applications of the technology, the present invention offers a superior solution to the aforesaid system. For example, in some environments sensors cannot be used.
It is therefore a general object of the present invention to provide an alternative method and apparatus to the aforesaid sensor system which is often an improved system from the perspectives of cost or reliability. The present invention does not require sensor components and moreover, will operate reliably and continuously for multitudes of cycles and within close tolerances to manipulate wafers from cassette holders to various process stations without any damage to wafers.
Another object of the invention is to provide an automatic calibration system for one or more degrees of freedom of a robot (namely R, .crclbar. or Z) while employing the aforesaid sensor system for the balance of the degrees of freedom. For example, the present invention could be used to automatically calibrate the R and .crclbar. axes of a given robot while the aforesaid sensor system could be used to calibrate the Z-axis.
Further objects of the invention are to provide a wafer-handling automatic calibration system that will automatically calibrate and adjust a wafer handling robot in a relatively short time, for example, after robot components have been removed and replaced.
Another object of the present invention is to provide an automatic calibration system that utilizes a motion control system that can digitally measure the velocity of a servo axis and, based upon this velocity signal, determine when the servo-driven axis begins to collide with the part of the target machine to which the robot is referencing.
Another object of the invention is to provide an automatic calibration system for a wafer handling robot which utilizes a machine controller that is programmed to utilize known dimensional data as well as robot motor velocity changes which occur when the robot touches structural features of a process station for controlling robot movements to precise wafer contacting locations.
Still another object of the invention is to provide a robotic wafer handling system having improved reproducibility of the position of the wafer holding wand in the locale where a semiconductor wafer is placed or removed from an enclosure by virtue of the known dimensional data of the wand and features of the enclosure as well as motor velocity inputs from the robot which are processed in a motor controller for controlling robot movement to precise wafer contacting locations.